The present invention is related generally to the correction of distortion of optical signals and, in particular, to the use of adaptive optics to correct that distortion.
There are nearly limitless uses for optical signals in many different fields for many different purposes. For example, such signals may be used in communications systems when analog or digital data is modulated upon an optical carrier signal, such as in an optical switch. Signals in such systems are then transmitted from one point to another using fiber optics or via free-space transmissions. Additionally, optical signals collected by telescopes are used in astronomy to view distant astronomical bodies and phenomena. There are also many uses for optical signals in the medical field. For example, by transmitting an optical signal into the human eye, it is possible to detect the light reflected off of the retina in that eye and then create an accurate map of the retina.
The operation of systems using optical signals may be hampered by a variety of factors. For example, distortion of a transmitted planar wave front of the light beam may occur due to any changes in the refractive properties of the medium through which the beam passes, including changes due to temperature variations, turbulence, index of refraction variations or other phenomena. This distortion may cause discrete sections of the wave front to deviate from the orthogonal orientation to the line of travel of the beam as initially transmitted. This distortion may result in significant degradation of the wave front at its destination. In free-space communications systems, any disturbance in the atmosphere between the transmission point and the receiving point may cause certain portions of the beam to move faster than others resulting in the aforementioned wave front distortion. The same is true in astronomical and medical uses. For example, when used to create a map of the human retina, wave front distortion does not typically result from atmospheric disturbance but, instead, results from the light beam passing first into, and then out of, the eye through its lens. The small imperfections on the lens and cornea distort the wave front of the beam much like the distortion seen in communications or astronomical uses. Whatever the particular use, the result is the same: distortion prevents a planar wave front of the beam from being received at its destination in phase.
Adaptive optics uses a wave front sensor to measure phase aberrations in an optical system and a deformable mirror or other wave front compensating device to correct these aberrations. Deformable mirrors change their shape in order to bring the reflected wave front into phase. Until recently, these mirrors were typically deformed via piezoelectric drivers, mechanical screws, or other well-known methods. In recent methods, however, a deformable mirror may be actuated by a technique wherein an array of electrodes is located in electrostatic proximity to that mirror in the optical system. Electrostatic proximity means, as used herein, that by placing a voltage across these electrodes, an attractive force is created between those electrodes and the mirror. This procedure is known as electrostatic actuation. By controlling the attractive force along different portions of the mirror surface, the shape of the mirror may be altered in a known way, thereby at least partially correcting for the wave front distortion. Another adaptive optics method involves using magnetic forces to attract or repel portions of a mirror.
Systems using such deformable mirrors, however, have significant limitations. For example, prior art adaptive optics systems relying on electrostatic actuation to correct the shape of a wave front cannot cause an attraction between a particular electrode and a discrete portion of the mirror in one instant, and then cause a repelling electrostatic force between that particular electrode and that same portion of the mirror the next instant, or vice versa. Additionally, the voltage necessary to create a given pressure at a particular location on the mirror is nonlinearly dependent upon the distance from the mirror to the relevant electrode. Since this distance is always changing (e.g., portions of the mirror would be drawn closer to the electrode when the mirror was actively deformed), the same voltage used in different instances could cause significantly different results in the shape of the mirror at a particular location. Deformable mirrors using magnetic force to alter the shape of a mirror in order to correct the shape of the wave front also have significant limitations. For example, such mirrors required electric coils that, when energized, created significant heat. This heat has the effect of rendering the mirrors unsuitable for certain uses (e.g., infrared imaging) and, in extreme cases, could result in undesirable thermal stresses to various components of the system.
The aforementioned problems related to wave front distortion correction are solved by the present invention. In accordance with the present invention, a bound charge is integrated within a mirror or in a material mechanically coupled with the mirror. A portion of the bound charge layer near a particular electrode in a first group of electrodes located in electrostatic proximity to the bound charge layer will alternately be attracted or repelled from that particular electrode by changing the sign of the voltage across that electrode. Thus, the mirror that is mechanically coupled to this bound charge layer will similarly be attracted or repelled from that particular electrode. In one embodiment, a second group of electrodes located on the opposite side of the mirror from the first group of electrodes may be used to define the electrical field and enhance the ability of a particular electrode in the first group to selectively attract or repel a particular portion of the mirror. It is advantageous to arrange the groups of electrodes in planes.